An external force imposed abruptly by earthquake or the like, or deficiency in yield strength stemming from deterioration has repeatedly caused an abrupt collapse of a structure, resulting in damage to lives and property.
A structure collapses in the following manner. Component members of a structure are ruptured due to excessive load or deficiency in yield strength. Resultant deterioration of stability of the overall fabric of the structure causes significant deformation to the shape of the structure, thereby causing a reduction in the internal space of the structure; i.e., structural collapse. In many cases of collapse of a building, floors fall down in a heap, like a stack of pancakes, or collapse. In many cases of collapse of an elevated bridge, bridge piers are ruptured, resulting in collapse of the bridge. Accordingly, it rupture can be controlled through reinforcement of various members of a structure, such as structural members, to thereby avoid deterioration of the overall structural stability even after the members are ruptured, possible damage to lives and property in and around a structure can be reduced.
Conventionally, in order to attain safety through avoidance of collapse of a structure, the following measures have been employed.
{circle around (1)} The cross section or the like of a structural member is determined such that the structural member is not ruptured upon imposition of a required load, which is predetermined in consideration of the structural member's own weight and an external force to be abruptly imposed.
{circle around (2)} When an assumed external force to be abruptly imposed after construction of a structure increases or when the yield strength of a structural member decreases due to deterioration or the like, the cross-sectional area or material strength of the structural member is increased. Alternatively, a high-strength member, such as an iron plate or carbon fiber, is disposed around a structural member to thereby enhance energy absorption capability (toughness) until the yield strength or rupture of the structural member is reached.
{circle around (3)} A seismic isolator is installed for a structure so as to decrease a seismic force to be imposed on the structure.
When a structure has been damaged by an external force imposed abruptly by earthquake or the like, the structure is tentatively evaluated for the degree of damage, and access to the structure may be forbidden, depending on the evaluated degree of damage. When an assumed seismic load is increased as a result of revision of design standard, an existing structure is subjected to antiseismic diagnosis, and antiseismic repairs or reinforcement is recommended in the case of a structure judged to run a high risk of seismic collapse.
However, the conventional measures {circle around (1)}-{circle around (3)} are based on a previously assumed level (a design value) of an external force to be imposed abruptly by earthquake or the like. When an external force in excess of the assumed level is imposed on a member, the member is ruptured, resulting in a failure to ensure the overall stability of a structure.
Naturally, expenses, time, and material required for carrying out the conventional measures described above do not reach a level involved in new construction of a structure, but do reach tens of percent of the level. Thus, in many cases, the conventional measures involve excessively high cost. Also, in many cases, the conventional measures require workers skilled in welding, installation of reinforcing bars, finishing, and the like. Hiring such skilled workers is difficult nowadays. Accordingly, even when an existing structure is known to involve a great risk of collapse due to deterioration, or because the structure is designed according to old standard or has been damaged by an external force imposed abruptly by earthquake or the like, in many cases, reinforcement of the structure has been unfeasible, for economic and physical reasons. In a certain case, after occurrence of disaster, such as earthquake, when an examiner(s) entered a damaged structure in order to tentatively evaluate the degree of collapse risk, an aftershock caused the structure to collapse, with the result that the examiner(s) were killed or injured. In another case, when dwellers and users entered a structure which was judged safe in view of minor damage, an aftershock caused the structure to collapse, resulting in heavy casualties.
FIG. 21 shows typical loads imposed on a column 1, which is a typical structural member, and a corresponding displacement. A load is imposed on an end portion of a member or is imposed on a member in a concentrated or distributed condition. A load assumes the form of a force or moment. FIG. 21 shows typical loads to be imposed. FIG. 22 shows the relationship between a load to be imposed on a member and a corresponding displacement as shown in FIG. 21, in relation to the conventional measures described above. As shown in FIG. 22, reinforcement enhances strength and/or toughness; however, there is no guarantee that the member can bear an upper load after a toughness limit is exceeded.
Specifically, in the case of a small range of deformation (within 2%-3%), the conventional measures described above enable a member to bear a load, to thereby ensure the overall stability of a structure. However, in the case of deformation in excess of the range, a mechanism for bearing a load is lost, resulting in rapid progress of deformation. As a result, collapse of the structure becomes unavoidable. For example, in an example of a column 1 shown in FIG. 24(a), tie hoops arranged within the reinforced concrete column 1 can bear a circumferential tensile force T and a shearing stress S induced by an axial force (a vertical force) P that falls within a tolerance and thus induces merely a small range of deformation (within several %). However, the shearing stress S causes a shear fracture of the column 1 with a resultant impairment in rigidity, or an excessive axial force causes rupture or dislocation of a tie hoop(s) with a resultant failure to bear the circumferential tensile force T. As a result, as shown in FIG. 24(b), deformation progresses rapidly, followed by complete collapse as shown in FIG. 24(c). In this manner, the aforementioned pancake-like destruction phenomenon unavoidably occurs. Also, when a member 15 assumes the form of a beam 16 as shown in FIG. 25, cracks 20 and the yield of a reinforcing bar(s) cause compression rupture of a portion enclosed by the dashed line in FIG. 25.
In the case where a large number of structures must be reinforced immediately after occurrence of an abrupt disaster, such as earthquake, or due to revision of the seismic standard, the conventional measures described above are unsuitable for promptly coping with the situation so as to secure safety.
In view of the above problems involved in the conventional measures, an object of the present invention is to provide a method and configuration of reinforcement which are applied, from the beginning, to various members including structural members of a newly constructed structure or are applied to various members including structural members of an existing structure so as to control rupture for delaying progress thereof and delaying expansion of a spatial rupture region, thereby avoiding complete loss of the load sharing capability of the members, which would otherwise result from local rupture of the members; i.e., thereby enabling the members to share a load with one another to such an extent as to avoid collapse of the structure even after the members are visibly deformed. Another object of the present invention is to practice economy in expenses, time, and material required for reinforcement work as compared with the conventional measures, thereby enabling prompt reinforcement of a large number of structures.